Semiconductor wafer fabrication techniques often require precise positioning of a wafer. For example, a fabrication step that includes ion implantation requires that the orientation of the crystalline lattice of the semiconductor material be known. As another example, there must be a precise alignment of a semiconductor wafer relative to a reticle or photomask for patterning a conductive layer to form signal paths along previously fabricated circuit structure on the wafer.
Typically, a semiconductor wafer includes a "wafer flat" or an indexing notch. A wafer flat or notch is an edge feature that is used to identify the orientation of the wafer. Orientation devices for aligning semiconductor wafers having wafer flats are described in U.S. Pat. No. 5,183,378 to Asano et al., U.S. Pat. No. 4,441,853 to Kosugi, U.S. Pat. No. 4,311,427 to Coad et al., and U.S. Pat. No. 3,997,065 to Jaksch.
Also known are "notch finders" that are designed to align the notches of semiconductor wafers contained within a cassette. The notch finder may have a small diameter stainless steel rod having a coating of polyvinyldenefloride. When the cassette is positioned on the notch finder, the edges of the semiconductor wafers contact the stainless steel rod. Rotation of the stainless steel rod causes the wafers to rotate within the cassette. When a notch of a rotating wafer reaches the stainless steel rod, the notch allows the wafer to drop slightly onto the rod. The stainless steel rod continues to rotate, but the notch-to-rod registration prevents further rotation of the wafer. Within a relatively short time, all of the wafers in the cassette are aligned.
A notch finder may include a mechanism for uniformly rotating the wafers following the notch-alignment operation. Rollers may be positioned on opposite sides of the stainless steel rod out of contact with the wafers during the notch-alignment operation. Then, the rollers may be moved upwardly to contact the edges of the wafers, lifting the notches away from the stainless steel rod. The rollers can then be rotated to relocate the aligned notches to a desired position.
Conventional notch finders operate well for their intended purpose. However, there are concerns relating to operation of the conventional notch finders. As previously noted, the stainless steel rod continues to rotate after the first notch is brought into registration with the rod. Rotation of the stainless steel rod against the stationary edges of the notches generates noises that are potentially disruptive to persons in the general vicinity of the notch finder. More importantly, the relative rotation between the stainless steel rod and the semiconductor wafer may generate particles that settle on the surface of one of the wafers. Particulate contamination will adversely affect the manufacturing throughput of the semiconductor fabrication process. Another concern is that as the rollers are moved upwardly to lift the wafers from the alignment bar, the lifting rollers may cause nonuniform rotation of the wafers. Even a minute amount of undesired rotation may cause difficulties during a subsequent fabrication step.
An object of the invention is to provide an orientation method and apparatus for aligning disk-shaped members, such as semiconductor wafers, having indexing notches. Another object is to provide such a method and apparatus that achieve the alignment in a nonparticulating manner having a small number of operating components, thereby providing cost benefits.